A semiconductor device as a RC-IGBT having an insulated gate type bipolar transistor device (i.e., an IGBT device) and a diode device, which are formed in a same semiconductor substrate, is well known. In the semiconductor device, an energization electrode (i.e., a collector and an emitter) of the IGBT device and an energization electrode (i.e., a cathode and an anode) of the diode device are provided by a common electrode. In the RC-IGBT, when a gate drive voltage is applied under a condition that current flows through the diode device, a channel is formed, so that a hole injection is restricted. Thus, the RC-IGBT has a characteristic that conduction loss increases.
Thus, while an on-state instruction signal is applied to the RC-IGBT, current flowing through a sense element in the RC-IGBT is compared with a threshold value, so that the RC-IGBT determines whether current flows through the diode device. When the current flows through the diode device, the gate drive voltage is shut off, and when the current does not flow through the diode device, the gate drive voltage is applied. These drive control methods are disclosed in JP-A-2010-118642 (corresponding to US 2010-0090248-A1).
It is necessary to perform the shut-off control of the gate drive voltage only when the current flows through the diode device. For example, in an inverter apparatus, when the gate drive voltage is shut off under a condition that the current flows through the IGBT device according to the on-state instruction signal, i.e., under a condition that the current does not flow through the diode device, a current waveform is deformed. Accordingly, high accuracy for determining a polarity whether the current flows through the diode device.
However, a current detection part such as a sense resistor includes a parasitic inductance, which is not avoidable. Accordingly, when an element current flowing through the RC-IGBT changes, a vibration is generated in a current detection signal attributed to the parasitic inductance. As a result, especially when the element current is small, a vibration component of the current detection signal falls below a threshold value. Thus, the apparatus may fail to detect a polarity. Further, the vibration may be overlapped on the current detection signal attributed to other reasons.
In view of the above failure, it is considered that the threshold value used in the polarity detection may be set higher so as to avoid the above failure. However, when the threshold value is high, a dead zone, in which the gate drive voltage is not able to shut off although the current flows through the diode device, expands. Accordingly, especially when the current is small, the conduction loss is not sufficiently reduced.